Present-day automatic transmissions for a front-transverse arrangement in a motor vehicle comprise either an upstream converter and a downstream, power-shift planetary transmission or a dual clutch transmission.
The installation space for this drive train arrangement is so limited that the arrangement of the required elements has to be as compact as possible to produce the transmission ratios. This pertains in particular to the installation length of the transmission, since the installation space for a transverse installation is very limited. Furthermore, the efficiency of the transmission is a very important criterion, whereas due to their configuration, dual clutch transmissions have an advantage over conventional planetary transmissions with regard to power losses due to both tooth friction and clutch plate slip. The reason for this is that in a dual clutch transmission, two gears must be engaged, whereas in a planetary transmission two gears must be engaged and in, addition, planetary transmission losses lessen the efficiency in comparison to a dual clutch transmission. Furthermore, a dual clutch transmission has lower slip losses than a planetary transmission, since in the case of a dual clutch transmission, one disengaging power-shift clutch is provided, in comparison to at least three disengaging power-shift clutches in the planetary transmission.
Planetary transmissions in a front-transverse design for passenger vehicles usually have a converter as start-up element and comprise at least two planetary gear sets and at least five power-shift elements, which usually are designed as clutch plates/brakes; as a rule, up to six forward gears and one reverse gear can be implemented. In this case, the planetary transmission can be positioned either fully coaxially to the drive shaft, or as a quasi-group transmission (as a rule, 5-gear transmission) with a primary transmission arranged coaxially to the drive shaft and one rear-mounted gear set arranged on a secondary shaft.
A transmission with a planetary design is disclosed in the document EP 0 434 525 B1, which comprises essentially a drive shaft and an output shaft that are arranged parallel to each other, a dual planetary gear set that is arranged concentric to the output shaft, and five shift elements in the form of three clutches and two brakes, whose optional locking in pairs determines the different gear transmission ratios between the drive shaft and the output shaft. In this case, the transmission has a front-mounted gear set and two power paths, so that due to selective, paired engaging of the five shifting elements, six forward gears can be realized.
A design comprising a secondary shaft is known, for example from JP 2005-23987. In this case, the two gear sets of the primary gear set are arranged on two parallel shafts and are actuated jointly by means of two spur gear-drives or two chain drives. The input torque is applied to the two stated shafts by means of a transmission input shaft actively connected to the two stated shafts by means of a spur gear drive, and the torque is transmitted to the pertinent elements of the planetary gear sets by means of the shifting elements according to the shifting logic.
Furthermore, from the prior art a dual clutch transmission with a countershaft gearbox design is known, in which the dual clutch is used as a start-up and shifting element; in this case, the countershaft gearbox can be positively shifted.
From document DE 10 2004 001 278 A1 a dual clutch transmission is known which has two input shafts, two clutches and at least one first and one second drive shaft, whereas the first input shaft can be actively connected to the first clutch and the second input shaft can be actively connected to the second clutch, and the first drive shaft is positioned below, and the second drive shaft is positioned above, the input shafts, or vice-versa, and the input shafts and drive shafts have mutually engaged toothed gears and at least two toothed gears form one gear stage.
In this case at least one toothed gear of a gear stage is designed as an engaging and/or disengaging loose wheel and the other toothed gear is designed as a fixed wheel; furthermore, each drive shaft has one driven toothed gear to transfer the torque to the axle drive, whereas to implement the reverse gear, an additional shaft is provided which is designed as a third drive shaft.
From the prior art dual-clutch transmissions are also known with an outlet-connected planetary transmission, however these are not suitable for a standard drive and also not for a front-transverse arrangement.
For example, document EP 1 422 441 A2 discloses a family of multiple gear planetary transmissions with a dual clutch. A transmission of this kind comprises an input shaft and an output shaft, a first, a second, a third and a fourth planetary gear set, whereas the planetary gear sets each have three elements, whereas a first shaft permanently connects a first element of the first planetary gear set to a first element of the second planetary gear set, a second shaft permanently connects a second element of the first planetary gear set to a second element of the second planetary gear set, a third shaft permanently connects one element of the first or second planetary gear set to a first element of the third planetary gear set and of the output shaft, and whereas a fourth shaft permanently connects a second element of the third planetary gear set to a first element of the fourth planetary gear set. Furthermore, a first clutch is connected to the input shaft and a second clutch connects the input shaft to a third element of the third planetary gear set.
The transmissions described in EP 1 422 441 A2 also comprise a first and a second torque transmission mechanism, such that the torque transmission mechanisms optionally connect elements of the first and of the second planetary gear set to the first clutch. Furthermore, a third and a fourth torque transmission mechanism are provided, these torque transmission mechanisms optionally connect elements of the third planetary gear set to elements of the fourth planetary gear set; a fifth and a sixth torque transmission mechanism also connect operational elements of the first or second planetary gear set to a fixed-housing element, whereas a seventh torque transmission mechanism detachably connects one element of the fourth planetary gear set to the fixed-housing element. Furthermore, an eighth torque transmission mechanism is provided which optionally connects one element of the first or the second planetary gear set to the first clutch, or one element of the fourth planetary gear set to the fixed-housing element.
Additional dual clutch transmissions with downstream planetary gear transmissions are known for a standard arrangement, for example, from EP 1 422 448 A2, EP 1 424 510 A2, EP 1 424 511 A2, EP 1 566 570 A1 and from EP 1 566 574 A1.
Known from the Applicant's prior application DE 10 2004 014 082 A1 is a dual clutch transmission with a planetary design, with several planetary gear sets, at least two friction-locked shift elements for shifting to different power paths in a force flux, and several positive shift elements for selecting of different transmission stages in the power path. In this case, the friction-locked shift elements and the positive shift elements are arranged between the shafts of the planetary gear sets, a housing and also a transmission input shaft and a transmission output shaft, so that changes to gear stages can be performed at least in one lower gear stage range by means of the friction-locked shifting elements without interrupting the traction. At least one of the friction-locked shifting elements is designed as a clutch, and the second planetary gear set, a third planetary gear set and a fourth planetary gear set form a 3-stage-5-shaft transmission unit which is designed with separate planetary gears.
An additional dual clutch transmission for a standard arrangement comprising a planetary transmission is described within the scope of WO 2005/050060 A1. It comprises a drive shaft and an output shaft which are connected together by means of at least two power paths. One power path has at least two transmission ratios and these transmission ratios are different. Furthermore, the drive shaft is permanently connected to the input shafts of the power paths.
The object of the present invention is to specify a power-shifting dual-clutch transmission which has a very compact design and is suitable for a front-transverse arrangement. Furthermore, the transmission according to the invention can be produced at low cost and has a small power loss.